Start with a Doran's ring for life, power and mana regeneration. Arrive at lvl 6, go back and take an Amplifying tome and get 2 Sight ward and 3 Mana potion to finish the lane phasing. When you have the money for Nashor's tooth you will buy it. Your phasing ap with yi is finish now, you can go ad. Start with boots for mobility, take Vampirique scepter for lifesteal and go BF when u can to do an Infinity edge. After, go rush Phantom dancer, Ravenous hydra, Statikk shiv and take an Gardian angel after having sold Nashor's tooth and Doran's ring. Now you are in god mode, u can two shot adc and apc ! :)

Don't forget yi is not tank, he is really good 1v1 but if they come gank you with a stun, you die.